1. Field of the Invention
The present invention generally relates to a planar antenna, and more particularly, to a multiple frequency band planar antenna.
2. Description of Related Art
As the wireless internet access technology continues to evolve, users are able to access the internet at a higher speed at a fixed place where an internet station is located, such as, a train station, a university, etc., within a wireless local area network (WLAN). As a result, the wireless notebook has become a mainstream product in the notebook market because it allows users to freely access the internet, compared with the traditional notebook with wire internet access. Recently, a WiFi wireless Local Area Network (LAN) has been developed, which operates at about 2.4 GHz and ˜5 GHz (this frequency is referred to as a communication carrier frequency modulated by data signals with any modulation technology, such as an orthogonal frequency division multiplex (OFDM) technology). However, the wireless WiFi LAN technology has some drawbacks which limit its usage to only the neighbourhood of the aforementioned fixed place. These drawbacks include, for example, a low capacity and a short communication range (about several hundred meters) for wireless communication carriers, which limits the users to access the internet to a limited range away from the Internet station. Currently, a wireless WiMAX communication technology (i.e. IEEE 820.16 standard) has been developed to overcome the drawbacks of the wireless WiFi LAN technology; that is, WiMAX allows wireless communication carriers to have a higher capacity and a longer communication range without a significant attenuation so as to make it feasible to access the Internet at any place in a metropolitan area in which a WiMAX metropolitan area network (MAN) is constructed. Moreover, currently, the wireless internet-access technology employs several frequency bands with their operating frequencies at 2.4 GHz, 3.5 GHz, 5.15˜5.35 GHz and 5.8 GHz, respectively. Among these frequencies, 2.4 GHz, 5.25 GHz and 5.8 GHz are applied in the WiFi LAN while 2.3˜2.5 GHz, 3.5 GHz, 5.15˜5.35 GHz and 5.8 GHz are applied in the WiMAX MAN. Accordingly, in response to the need for both WiFi LAN and WiMAX MAN applications, a planar antenna with its operating frequencies at least including 2.4 GHz and 5.15˜5.35 GHz can be a suitable one. This broad-band antenna is referred to as a multiple frequency broad-band antenna.
Furthermore, a planar antenna is widely employed in the wireless communication technology because it is easily integrated with a printed circuit board (PCB), which, for example, is a glass-epoxy or Teflon-glass circuit board, so as to achieve compactness and low cost. For example, U.S. Pat. No. 6,535,167 B2 disclosed a laminate pattern antenna capable of operating at a wider frequency band. The laminate pattern antenna comprises an inverted-F-shaped antenna pattern formed as a driven element on the obverse-side surface of a PCB, and an inverted-L-shaped antenna pattern formed as a passive element on the reverse-side surface of the PCB. By virtue of setting a path length of the inverted-F-shaped antenna pattern to a specific value, this antenna makes the low-frequency side of its usable frequency range shift to the low-frequency side. Likewise, by virtue of setting a path length of the inverted-L-shaped antenna pattern to another specific value, this antenna makes the high-frequency side of its usable frequency range shift to the high-frequency side. As a result, the laminate pattern antenna is able to operate at a wider frequency band; however, its operating frequency is about 2.4 GHz, which limits its application to only WiFi LAN, but not WiMAX MAN. Besides, as the laminate pattern antenna has a complicated structure, its fabricating procedures are accordingly lengthy because they comprise procedures for forming the inverted-F-shaped antenna pattern and then the inverted-L-shaped antenna pattern on both surfaces of the PCB, which in turn increases a fabricating cost. Accordingly, the laminate pattern antenna fails to meet a compactness requirement of a planar antenna due to its laminated structure, in addition to its narrow frequency band. Hence, the design of a novel pattern planar antenna that has multiple frequency bands, a simple antenna structure and a low fabricating cost is highly desired.